represent uid/gid as uint32, not uint16
represent uid/gid as uint32, not uint16

this allows uid pools higher than 64k to work, as is often hit when
running Warden on each node of a test suite

e.g. uid pool start = 10000 * parallel-node

Signed-off-by: Simon Leung <23f08a50fbaf82c4ecfae6738d41887b087c2e71@us.ibm.com>

shashidharatd/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux